Turnbull's blue \Turn"bull's blue`\ (t[^u]rn"b[.u]lz bl[=u]`).
(Chem.)
The double cyanide of ferrous and ferric iron, a dark blue
amorphous substance having a coppery luster, used in dyeing,
calico printing, etc. Cf. {Prussian blue}, under {Prussian}.
[1913 Webster]